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Effect of biopolymer addition on the properties of rennet-induced skim milk gels : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Massey University, Palmerston North, New Zealand

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Abstract

The main objectives of this study were to determine the effect of adding different biopolymers (κ-Carrageenan, xanthan gum, guar gum, high-methoxyl pectin and gelatin) on the properties of rennet skim milk gels. A collection of techniques, namely strain-controlled rheometery, spontaneous whey separation measurements, confocal laser scanning microscopy and diffusing wave spectroscopy, were used. The effects of these biopolymers were investigated for rennet skim milk gels made under model system and cheesemaking conditions. However, only rheological measurements were performed for samples made under cheesemaking conditions. For samples made under model system conditions, the concentration of the biopolymer was varied from 0 wt% to 0.1 wt%. Experimental conditions, such as renneting temperature (30°C), total milk-solids (10 wt% reconstituted skim milk), pH 6.7 and rennet concentration (200 µL per 100 g sample) were kept constant. The rheological behaviour of these samples was affected by the addition of κ-carrageenan, xanthan, guar, high-methoxyl (HM) pectin and gelatin. Both rheology and diffusing-wave spectroscopy (DWS) showed that the aggregation and gelation time and the gel strength was affected by the addition of these biopolymers. It was also shown that the syneresis behaviour, as well as the microstructure of rennet gels as imaged by confocal laser scanning microscopy (CLSM), was altered upon adding these biopolymers. The rheological and microstructural properties of model renneted skim milk systems improved by adding small amounts (0.025 wt%) of κ-carrageenan, guar, HM pectin and gelatin, but not xanthan. Renneted skim milk containing HM pectin and gelatin had higher G*, decreased aggregation time and gelation time and lower syneresis values as the concentration of biopolymer was increased. On the other hand, lower G* and higher syneresis values were obtained for samples containing higher concentrations (> 0.025 wt%) of κ-carrageenan, xanthan and guar gum. Higher syneresis index was a consequence of the presence of larger pores in these samples, as shown from the CLSM micrographs. The effects caused by the addition of κ-carrageenan, xanthan and guar gum were believed to be due to phase separation in rennet skim milk gels containing polysaccharide, and was explained in termsof a depletion-flocculation mechanism. For rennet gels made under cheesemaking conditions (pH 6.2 with addition of 0.68 mM CaCl2), it was found that the addition of xanthan, guar, HM pectin and gelatin had similar effect to that when added to samples
made under model system conditions. This was due to the fact that the differences in pH and salt were known to not affect the properties of the biopolymers. However, the addition of κ-carrageenan, which was very sensitive to ions such as calcium, improved the viscoelastic properties of rennet skim milk gels made under cheesemaking conditions. Overall, this work provides useful information on the effects of adding κ-carrageenan, xanthan, guar, high-methoxyl pectin and gelatin on the properties of rennet-induced gels.